A commonly used technical Standard for video coding is known as H.264 (see Non Patent Literature (NPL) 1). The H.264 Standard includes a coded picture buffer (CPB) and a decoded picture buffer (DPB). The CPB is a first-in first-out buffer containing access units in decoding order specified in a decoder. The DPB is a buffer holding decoded pictures for reference, output reordering, or output delay specified for the decoder. The H.264 Standard uses an approach whereby one of two schemes that specifies which reference pictures are to be removed from the DPB. The first scheme (“sliding window reference picture marking”) specifies a first-in first-out mechanism for marking reference pictures as “unused for reference”. The second scheme (“adaptive reference picture marking”) relies on specific syntax elements in the video bit stream to specify the marking of selected reference picture(s) as “unused for reference”.
There is in development a draft Standard on high-efficiency video coding (HEVC), known as ITU-T SG16 WP3 and also known as ISO/IEC JTC1/SC29/WG11. A buffer description (BD) (also known as a reference picture set (RPS)) has been proposed. The BD describes the available reference pictures in the DPB at the start of the encoding/decoding process of a target picture. The basic concept of the BD is to signal information for keeping/retaining reference pictures in the DPB.
More particularly, the BD is a list of all reference pictures that are stored in the DPB. Each item (corresponding to one reference picture) in this list is referred to as a buffer element (BE). A BE comprises a unique picture identifier such as a picture order count (POC) number and additional picture information such as a usage flag indicating the temporal layer/level of the picture. The usage flag indicates whether or not a reference picture is used in inter-picture prediction coding of a current picture. When a reference picture is not used, it remains in the DPB and may be used in inter-prediction coding of future pictures following the current picture in coding order. Inclusion of the usage flag in the BD is useful for error robustness, as the flag is required in constructing reference picture list(s). If a reference picture is lost (e.g. due to transmission errors), reference picture lists of subsequent pictures can still be constructed as intended by the encoder.
A BD is activated and applied at the start of the encoding/decoding process of a target picture. Pictures in the DPB that are not included in the active BD are considered as non-reference pictures and are marked as “unused for reference”. The pictures marked as “unused for reference” may still be kept in the DPB for a bumping scheme such as the scheme specified in the AVC video coding scheme.
The bumping scheme removes/deletes pictures from DPB. Even when a picture is not a reference picture, they may need to be kept in the DPB until its output/display time instance.
A reference picture list is constructed for encoding/decoding a target picture. Each entry in the reference picture list is identified using a reference index. In H.264 and HEVC provides the flexibility where not all valid reference pictures are listed in a reference picture list and a valid reference picture may be present in more than one entry in a reference picture list.